Display issue for new iMac Big Sur

I bought the brand new one 2 days ago, it everything was perfectly until today. I upgraded yesterday for the new version 11.5.1, it was good as well. But I didn't know what I did that the display totally destroy, it went so vivid and exposure, colour so deed and bright, I spoken to online tech support, they could not figure out what happened, the agent John helped me created the new user to solve the problem. The question is would be okay any one help me to figure it out, so I don't have to rebuild everything?